Abstract: Geochemistry of Light Hydrocarbons in Shallow Holes, Atlantic Margin Coring Project--Preliminary Results

R. E. Miller, D. M. Schultz

Light hydrocarbons (methane, ethane, and propane) were detected in different concentrations in 8 of the 20 holes cored during the course of the Atlantic Margin Coring Project (AMCP). The concentrations of methane in sections of sediments ranged from less than 100 ppm to more than 400 × 10³ ppm. Ethane values ranged from less than 50 ppm to more than 10 × 10³, whereas the propane concentrations were generally less than 800 ppm.

Highest concentrations of the light hydrocarbons were generally in Pleistocene upper Continental Slope sediments. Preliminary interpretation suggests that these gases are dominantly of biologic origin, resulting from microbial decomposition of organic matter in anoxic depositional environments, where relatively high rates of sedimentation under deltaic-type conditions prevented oxidation of the organic matter.

Most of the high-concentration gas intervals in the cores were associated with zones of high organic productivity, as indicated by abundant diatom oozes. However, an exception to this association was in corehole 6021 (Mid-Atlantic upper Continental Slope, New Jersey region), where a 700-ft (213 m) section of Pleistocene sediments contained concentrations of methane in excess of 290 × 10³ ppm without evidence of microfossil oozes. Ethane increased slightly relative to methane in the lower section of this core, and hydrocarbons heavier than propane were not detected.

Consideration should be given to the assessment of the foundation stability of the upper Continental Slope sediments prior to construction and installation of permanent platform structures, as the gas content of these sediments could constitute an environmental-geologic hazard.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC